MB, probably something in that.
The officer pool is comprised largely of JO and SO, the worker bees if you like and essential to any organization to staff the organisation. Now in the military the VSO are drawn entirely from this pool. The gene pool is small compared with industry or civil service (which can draw from industry).
The up or out amongst VSO is certainly necessary to thin the upper ranks. If you want to serve to 55 stay as a wg cdr.
Met one OC Eng, amazing CV, oil exploration in Saudi, worked in Antarctica, then joined RAF. Reached OC Eng by 40, and said there is nothing I can see for me in the RAF now. Off he went.